1. Field of the Invention
This invention relates to ethylene/pentene-1 copolymer compositions having an excellent balance between impact resistance and tear properties, excellent transparency and good hot tack properties, and are particularly useful for packaging films.
2. Discussion of Related Art
Polyethylene prepared by high-pressure process is known as one of the polyolefin resins having relatively high transparency, and has been widely used for application in films or hollow containers. Speaking about the use of polyethylene in film, however, the high-pressure polyethylene, when molded into film by air-cooled inflation process often used in common film formation, cannot give products having sufficient transparency, impact resistance and tear resistance. There has been adopted a process for the copolymerization of ethylene with other polymerizable monomers such as vinyl acetate in order to improve these drawbacks. This process, however, undesirably brings about such problems that the film resulting from the ethylene copolymer prepared thereby decreases in mechanical strength and rigidity, or said film is liable to undergo blocking, thereby throwing hindrances in the way of the molding operation.
By the way, a copolymer of ethylene and .alpha.-olefin of at least 3 carbon atoms prepared by using a Ziegler catalyst is known as a resin excellent in mechanical strength and having a density nearly equal to that of the high-pressure polyethylene. Generally, ethylene/.alpha.-olefin copolymers prepared by using a vanadium-based catalyst as the Ziegler catalyst have a low melting point, and hence heat resistance and mechanical strength come into question.
An ethylene/.alpha.-olefin copolymer prepared by using a mixture of a titanium-based solid catalyst with an organoaluminum compound catalyst component as the Ziegler catalyst, on the other hand, has a high melting point and excellent heat resistance in comparison with the above-mentioned ethylene/.alpha.-olefin copolymer prepared by using the vanadium-based catalyst. However, when this ethylene/.alpha.-olefin copolymer prepared by using the above-mentioned mixture as the Ziegler catalyst contains structural units derived from propylene of 3 carbon atoms or butene-1 of 4 carbon atoms as the .alpha.-olefin, the mechanical strength of said copolymer comes into question. On that account, the .alpha.-olefin used in this ethylene/.alpha.-olefin copolymer is selected from among .alpha.-olefin having at least 6 carbon atoms in order to obtain sufficient mechanical strength of the resulting copolymer. Although a film obtained from a copolymer of ethylene and .alpha.-olefin having at least 6 carbon atoms is excellent in impact strength, the film has such problems that because of its tear strength higher than that required, it does not tear easily and becomes poor in tear properties (poor in peeling properties).
The present inventors have already found that a resin having both excellent impact strength and appropriate tear strength, i.e. excellent tear properties, may be obtained by copolymerizing ethylene and pentene-1 of 5 carbon atoms using a titanium solid catalyst component and an organoaluminum compound catalyst component.
The resulting resin, however, has still insufficient hot tack properties which are an index of resistance to peeling strength applied to a heat sealed part thereof directly after heat sealing, and therefore sufficient attention must be paid to the control of a packaging speed, appropriate temperature, etc., when the resin is used for automated filling and packaging. The resin tends to have insufficient transparency when polymerization is carried out with some catalyst systems. There is, therefore, still room for improving the problems as mentioned above.